1. Field of the Invention
The present invention relates generally to systems, methods and apparatuses useful for well drilling sites, and well site analysis. More specifically, the present invention relates to systems, methods and apparatuses for well site analysis using wire line formation evaluation tools for in situ determination of critical desorption pressures in water saturated coal bearing formations.
2. Background of the Invention
Wire line formation evaluation tools have been used for over 50 years in conventional formations to acquire formation pressures, permeability, fluid samples and many other reservoir characteristics. These same wire line tools have been used successfully to characterize coal bed methane formations.
Coal bed methane reservoirs can contain a quantity of gas adsorbed onto the coal surfaces and dissolved in formation water. The quantity of the sorbed gas within the coal matrix and dissolved in the water contributes a partial pressure to the overall pressure of the reservoir. Due to known relationships between gas partial pressures, and gas solubility concentrations, determining the partial pressure of the gas within the reservoir provides an effective method to determine the overall gas content of the reservoir. Once gas content is quantified, operators are better able to make informed decisions about the economic feasibility of well site operations and drillings.
The effective partial pressure of the natural gas can be found by determining the critical desorption pressure of the adsorbed gas. Since the adsorbed gas on the coal surfaces and the dissolved gas in the formation water exist in equilibrium, the critical desorption pressure is the pressure at which the pressure inside the reservoir is equal to the partial pressure of the dissolved gas. At the critical desorption pressure, the reservoir begins to discharge the dissolved gas as bubbles in the water. The critical desorption pressure is the formation pressure at which a water saturated coal would have to be reduced to for desorbed gas production to begin. Because water pump out and disposal costs during well production can be significant, determination of the critical desorption pressure is a key factor in defining the economics of a coal bed methane formation.
Determination of the critical desorption pressure is typically performed by retrieving a core sample from the well site. Once brought to the surface, the core sample is sealed into a container to prevent the escape of any gas prior to analysis. The core sample is then allowed to liberate its gas under controlled conditions, so that the total quantity of gas can be measured. The gas produced is then related to the critical desorption pressure using known isotherms.
However, there are several sources of error using the known procedure. Coal cores begin depressurization and gas desorption as they are brought to surface before they can be contained in a canister and therefore errors are introduced before laboratory measurements can be made. Gas released from the core sample before it is contained must be estimated, introducing a source of error in the determination of the critical desorption pressure. Additionally, the core sample must be taken away from the drill site to an off-site laboratory for analysis. Waiting on off-site laboratory determination of the critical desorption pressure can often take days, thus delaying the drilling process.